This invention relates generally to bearing units for rotatable shafts which are subject to high loadings. More particularly, the present invention is directed to devices for pre-loading a pair of axially spaced radial bearings for a rotatable shaft.
There are a number of applications wherein a cutting or grinding member, such as a disc in a disc mill or refiner, is mounted to a rotatable shaft and wherein during the operation, the shaft is subject to very large loads which may have significant axial and radial components relative to the rotational axis of the shaft. Shaft bearing units which permit efficient rotational operation of the shaft without substantial play under intense loading conditions are critical components of any such shaft assemblies. It is common to employ ring-like bearing units which may have conical, spherical or other shape bearing element configurations as radial bearings.
It is well-known that in order to achieve a substantially play-free shaft/bearing configuration, the bearing unit must be adapted to take into account both axial loading as well as radial loading. The axial loading on the shaft is often proportional to the radial loading. One conventional proposal to account for the axial/radial loading conditions is to incorporate tandem radial bearing units which are oriented in opposed relationship to the shaft. The outer rings of the bearing units are pre-loaded against each other by a spring assembly. While such units have proved to be advantageous, such units are substantially play-free in the axial direction. However, for many applications a limited axial displacement may be desirable for a number of reasons, including the accommodation of thermal expansion, which may be not insignificant when the shaft is subjected to intense loading conditions.
Some constructions have addressed the axial displacement problem by mounting the bearings within a bearing housing. Under certain conditions, the bearing housing is axially displaceable within a second primary housing.
Another approach to allowing for limited axial displacement is to mount the radial bearings in a separate bearing seat. A pre-loading device is disposed between the bearing seats to allow limited axial displacement in the bearing housing. The bearing seats are thus axially displaceable within the stationary bearing housing assembly.
For conventional devices, wherein the pre-loading force is implemented by spring units, the pre-loading force can vary during operation, especially as a result of varying operating temperatures. Consequently, if the bearings are not in close proximity, the spring pre-loaded bearing units may represent an impractical design approach. It is also established that in most applications, the applied radial loads can vary throughout the operation. The variability in the axial loads also may result in an associated axial pre-loading which increases or decreases proportionally.
In some applications, bearings for the shaft are pre-loaded by hydraulic systems. For example, U.S. Pat. No. 3,574,424 discloses an axial loaded bearing wherein a pair of axially spaced bearings are axially loaded by a circulating oil pressure system.